This invention relates to a method of forming a connection between a tube and a fitting, and to a connection between a tube and a fitting.
So-called compression fittings are known, in which a female threaded member through which a tube extends, is tightened onto a male thread of a body of the fitting, the male thread being provided on an exterior surface of a tube receiving formation of the body into which formation an end of the tube is received. Tightening of the female threaded member causes deformation of an annular compression means positioned between the body and the female threaded member to form a seal with the tube. The body may be a straight or elbow tube connector for example, a valve, gauge or any other fitting to which it is required to connect a tube.
For high pressure installations, i.e. installations in which the fluid pressure in the tube may be in the order of 10,000 psi, the compression means may be first and second interacting compression elements. Typical installations in which high pressure tube fittings are used, are in the oil, gas or chemical industries.
Typically, the first element is urged into engagement between an internal surface of the tube receiving formation, and the tube, by the second element upon which a radially inwardly extending shoulder formation of the female threaded element bears as the female threaded element is tightened onto the body.
Such high pressure tube fittings have been proven by the test of time, to operate adequately, although there are potential problems.
First, tightening of the female threaded element an amount to conform to design specification, is critical to correct operation of the fitting. Overtightening of the female threaded element can lead to the first element causing tube damage, which can result in sudden and dangerous tube failure under pressure. Undertightening can result in fluid leakage, or more seriously, total failure of the connection, again with potentially dangerous consequences.
Thus it is usual for the amount of rotation of the female threaded member relative to the body to achieve a predetermined tightness beyond finger tightness, to be specified. Judging finger tightness and the amount of rotation in sometimes hostile environments in which such high pressure fittings are used, can be very difficult, but in any event, a correct tightening operation relies entirely on the skill of the fitter.
Second, particularly when such connections are repaired or replaced, for example when an installation is modified, it is not unknown for one or more component parts of one manufacturer""s tube fitting to be replaced with corresponding component parts from another manufacturer. Because different manufacturers produce tube fittings in which the component parts are not designed to be mixed with component parts of another manufacturer, or at least it has been shown by work carried out by the inventors that component parts from one manufacturer should not be used in conjunction with other than component parts made by the same manufacturer, interchanging component parts can again lead to inadequate and potentially dangerous connections in high pressure applications.
According to one aspect of the invention we provide a method of forming a connection between a tube and a fitting, the fitting including a body with an externally male threaded tube receiving part to receive a tube, and a female threaded member adapted to be engaged with the male thread of the body with a compression means therebetween, the method including the steps of arranging the female threaded member and the compression means around the tube, inserting the tube into the tube receiving part of the body, and tightening the female threaded member onto the tube receiving part so that a generally radially inwardly extending formation of the female threaded member bears upon the compression means, the method being characterised in that the method includes determining the amount of movement of a part of the fitting which, as the female threaded member is tightened is caused to move relative to another part of the fitting.
The method may include monitoring relative movement of the part as the female threaded member is tightened, and continuing to tighten the female threaded member until the part has relatively moved a predetermined extent.
Thus by calibration to determine a predetermined extent of relative movement of the part of the fitting corresponding to a desired male thread/female threaded member tightness, by the method of the present invention correct tightening of the female threaded member can be more consistently achieved resulting in more reliable and thus safer connections.
Moreover, monitoring of the deformation of the part of the fitting may reveal that component parts of the connection from different manufacturers have been used, for example if the predetermined extent of movement cannot be achieved, or is achieved without an adequate or corresponding rotation of the female threaded member.
The invention is particularly but not exclusively concerned with a high pressure fitting, by which we mean a fitting adapted for use in installations where fluids having pressures of at least 100 psi pass through the connection.
The compression means may thus include first and second annular compression elements, the second element interacting with the first element as the female threaded member is tightened, to urge the first element into engagement between the tube receiving formation and the tube.
In one arrangement, the part the amount of relative movement of which is determined, is the generally radially inwardly extending formation of the female threaded member which when the female threaded member is tightened, moves relative to the remainder of the female threaded member generally axially of the connection. In this case, in order that measurable deformation of the part of the fitting occurs, the method may include providing the female threaded member of the fitting with a weakened region located such that as the female threaded member is tightened, movement of the formation is enhanced.
Preferably though, the weakened region is provided so as only to permit deformation of the formation within the elastic limit of the material from which the fitting is made.
The weakened region may include an annular groove provided in a region of the female threaded member where the radially inwardly extending formation joins the remainder of the female threaded member. Such groove is preferably provided in an internal surface of the female threaded member, e.g. axially adjacent to the thread.
In another arrangement part the amount of relative movement of which is determined, is an axially extending portion of the female threaded member which moves axially of the connection when the female threaded member is tightened. Thus the invention may include axially stretching the female threaded member relatively to move the axially extending portion thereof.
In this case, the method may include forming a circumferential groove in an external surface of the female threaded member, and determining a change in the axial length of the groove arising as a result of tightening of the female threaded member.
In yet another example, the part the amount of relative movement of which is determined, is a portion of the compression means, such as a rear portion of the second annular compression element on which the radially inwardly extending formation bears, which as the female threaded member is tightened, is deformed to move into a space between the female threaded member and the tube.
Thus the method may include providing a circular opening in the female threaded member of a first diameter, to receive the tube, which diameter is greater than is required to receive the tube, to provide an enlarged space into which the rear portion of the second annular compression element may deform, to facilitate movement of the part.
The part, the amount of relative movement of which is determined may be a portion of the compression means which moves axially towards the body as the female threaded member is tightened and may be a portion of the first or second annular compression element.
The method may include forming an opening in the female threaded member at a position such that movement of the portion of the compression means which moves as the female threaded member is tightened may be monitored. Such opening may be provided in a side wall of the female threaded member, or in the generally radially inwardly extending formation.
Determining the amount of relative movement can be performed by any one of or a combination of methods, including optically determining the movement of the part of the fitting, for example using an optical instrument which produces a light beam which is deflected or reflected to measure movement of the part, ultrasonic means acoustically to measure such movements for example using an instrument which produces ultrasonic vibrations which are reflected from the part of the fitting, the nature and/or direction of the reflections changing as the part moves, or even a mechanical measuring instrument which mechanically measures the amount of movement of the part.
In each case preferably the instrument is operated in conjunction with a tool which is used to tighten the female threaded member. For example, the tool and the instrument may be integrated.
According to a second aspect of the invention we provide a method of monitoring a connection formed by the method of the first aspect of the invention characterised in that the method includes providing data including a measurement of the amount of movement of the part of the fitting achieved during tightening of the female threaded member, to a data storage means, reexamining the connection to determine any change of position of the moveable part occurring since tightening.
Thus in critical situations, such as critical connections in a nuclear power plant or where toxic gases or liquids are being contained, long term monitoring of the connection can be carried out.
The method may include providing the connection with a monitoring means which provides a signal to the data storage means in the event that a movement of the movable part subsequent to tightening, beyond a threshold amount, is determined. The threshold amount may be determined by the detection ability of the monitoring means, or the monitoring means may determine intelligently, whether or not to send a signal to the data storage means.
Preferably the data storage means includes a processing means and thus has computing power, and the method includes providing a warning in the event that a movement of the moveable part subsequent to tightening, beyond a threshold amount, is determined, thus to give early warning of potential connection failure.
According to a third aspect of the invention we provide a connection between a tube and a fitting for use in the method of the first aspect of the invention, the part of the connection which is monitored including a weakened region, which enhances deformation of the part of the fitting within the elastic limit of the material from which the part of the fitting is made, as the female threaded member is tightened. According to a fourth aspect of the invention we provide a connection between a tube and a fitting for use in the method of the first aspect of the invention, characterised in that an opening is provided in the female threaded member through which movement of the part can be determined.